In most of applications of switch power, there is a need of electric insulation between the input and the output to protect a user from a danger of an accident due to high voltage or leakage current. A high-frequency transformer is used for the insulation and this converter is called an ‘isolated DC-DC converter’. The transformer adjusts the magnitude of the output voltage, using the ratio of the first and second cables, other than insulating. Typical isolated converters are a flyback converter, a forward converter, a push-pull converter, a half-bridge converter, and a full-bridge converter. The flyback converter and the forward converter are generally used for small power circuits under hundreds of watts.
FIG. 1 is a circuit diagram of a flyback converter of the related art. Referring to FIG. 1, the flyback converter includes a flyback driving unit 100 that supplies a primary current by turning on/off an internal power switch, a transformer 200 that receives the primary current and outputs a secondary current in accordance with the turn ratio of a primary cable and a secondary cable, a rectifying diode 300 that rectifies the secondary current, an output capacitor 400 that smoothes voltage through the rectifying diode 300, and an output resistor 500 that is connected with a load.
The flyback converter is the same in basic operation as a buck-boost converter of non-isolated converters. When power is supplied to the switch in the flyback driving unit, a current flows to the primary cable of the transformer and an input voltage is induced in the cable. A voltage with a polarity opposite to that of the primary cable is induced in the secondary cable due to the direction of the black spot, such that the diode is reversely biased and the current is stopped, and thus energy is accumulated only in the magnetizing inductance of the primary cable. When the switch is disconnected, a voltage having a polarity opposite to that of the previous state is induced in the secondary cable and power is supplied to the diode, such that the energy accumulated in the magnetizing inductance of the transformer is discharged to the output part.
In this configuration, the transformer is the most essential part that changes the magnitude of an AC voltage, using electromagnetic induction. When a coil is wound at both sides of a metal core and the current from one power supply changes with the lapse of time, the magnitude of the magnetic field changes accordingly. The magnetic field is transmitted through the metal core, so the magnitude of the magnetic field passing through the coil at the opposite side also changes with the lapse of time. At the opposite coil, an induced electromotive force is generated by electromagnetic induction and an induced current flows, such that an AC current is induced. The transformer can change the magnitude of a voltage in accordance with turn ratio of the coils. An important performance index in power transformers is a coupling coefficient. That is, the loss due to leakage inductance in the existing transformers depends on the coupling coefficient.
The existing transformers generally use cables formed in a sandwich type. The gap between the primary cable and the secondary cable should be small to achieve a transformer having excellent coupling, but when cables are formed in a sandwich type, there is a limit in reducing the gap between the cables. In the existing transformers using a common coaxial cable to solve this problem, not a solenoid cable, but a loop cable is formed around the center space of a toroidal core, because the cables are thick. This configuration imposes a burden on the prices, so there is a defect that it is generally used only for large capacity and the coupling coefficient is not high.
A transformer using a coaxial cable which has a small gap between the primary cable and the secondary cable is described in detail hereafter with reference to FIGS. 2 and 3.
FIGS. 2 and 3 are views illustrating the structure of a transformer using a coaxial cable according to the related art.
As illustrated in FIGS. 2 and 3, the transformer of the related art has a structure formed by winding an inner conductor and an outer conductor on a bobbin 30 as a primary cable 10 and a secondary cable 20 (the opposite is possible), using a coaxial cable, and then enclosing it with a magnetic core 40. The transformer using a coaxial cable of the related art has a problem in that it is difficult to be used for high-voltage power devices, because the turn ratio of the primary cable 10 and the secondary cable 20 is necessarily 1 to 1. Accordingly, several pieces of one-to-one cables are connected in series between the inner conductors and in parallel between the outer conductors in the transformers of the related art in order to achieve various turn ratios, but it causes the efficiency to reduce.
The background of the present invention has been disclosed in U.S. Pat. No. 4,814,965 (Mar. 21, 1989).